If you’re thinking about replacing your Weathershield TDL (True Divided Lite) sash windows with SDL (Simulated Divided Lite) ones, you’re probably looking for a way to improve energy efficiency while keeping a classic look. It’s a common update in older homes—and for good reason. SDL windows give the same traditional appearance as TDLs but with fewer maintenance issues and better insulation.

In this guide, I’ll walk you through everything you need to know to make the switch—from understanding the differences between TDL and SDL to step-by-step instructions on how to replace the sash yourself.

What’s the Difference Between TDL and SDL?

Let’s start by clearing up what sets these two types of windows apart.

TDL (True Divided Lite): These windows are made from multiple small panes of glass held together by wood muntins. They have that charming, old-school look and are commonly found in historic or traditional homes.

SDL (Simulated Divided Lite): SDL windows, on the other hand, use one large pane of glass with grilles applied to the surface to give the illusion of separate panes. There’s often a spacer bar inside the glass, which makes the illusion even more convincing.

The main advantage of SDLs is better energy performance. Since there are fewer breaks in the glass, they’re more airtight and better insulated.

Why Switch from TDL to SDL

There are a few reasons people choose to make the switch:

Lower Energy Bills: SDLs are more efficient at keeping warm or cool air inside your home.

Less Maintenance: Fewer parts mean fewer things that can break or leak.

Modern Materials: SDL sashes tend to hold up better over time.

Still Looks Traditional: You don’t have to sacrifice that classic divided-lite look.

So if you love the look of your current windows but want something easier to manage, switching to SDL might be the perfect balance.

What You’ll Need

Before you start the replacement, gather your tools and materials:

A replacement SDL sash (make sure it matches the size and style)

Tape measure

Screwdriver or pry bar

Utility knife

Silicone caulk and a caulk gun

Wood filler or paint (if needed)

Gloves and safety glasses

Step-by-Step: How to Replace a TDL Sash with an SDL Sash

Step 1: Measure the Existing Sash

First things first—grab a tape measure and get accurate dimensions of your current sash. Measure the height, width, and thickness. Don’t rely on old paperwork or product info; actual measurements are always better.

Write everything down. You’ll need these numbers to order your replacement SDL sash.

Step 2: Order the New SDL Sash

Once you’ve got your measurements, place an order for an SDL sash that matches not just the size but also the style and grille pattern of your current windows. You want everything to line up visually so the new sash blends in.

Step 3: Remove the Old TDL Sash

Removing the old sash can be a bit tricky if it’s been painted over or sealed tightly. Start by unlocking the window and raising it slightly. Use a flathead screwdriver or pry bar to gently loosen the stops or clips that hold it in place.

Tilt the sash inward and lift it out. Be careful not to damage the frame or surrounding woodwork.

Step 4: Check the Frame

With the sash out, now’s a good time to check the frame. Look for any signs of water damage, rot, or wear. If anything looks off, take care of it before installing the new sash. It’s better to fix these issues now than deal with them later.

Step 5: Test Fit the New SDL Sash

Before you commit, do a dry fit. Put the new sash into the frame and check that it fits properly. It should slide and tilt smoothly and line up with the existing frame.

If anything feels too tight or misaligned, now’s the time to make adjustments—not after you’ve sealed it in.

Step 6: Install the SDL Sash

Once everything lines up, go ahead and install the sash. For double-hung windows, tilt the bottom in first, then the top. Make sure everything is seated correctly.

If your window uses sash cords, weights, or balance systems, reconnect those at this point.

Step 7: Seal the Edges

To keep drafts out and improve energy efficiency, apply a bead of silicone caulk around the perimeter of the sash. Use a steady hand to get a clean line. If you notice small gaps or imperfections, you can use wood filler and touch-up paint to finish things off.

Step 8: Final Touches

Clean up any mess, remove excess caulk, and give the window a once-over. Make sure it opens and closes smoothly, locks properly, and looks good from inside and outside.

If you replaced just one sash, stand back and compare it to the rest. If the match is good, you’re all set. If something’s off, you may want to adjust the next order or repaint for consistency.

Helpful Tips

Consistency Is Key: If you’re only replacing one or two sashes, make sure the grille pattern and frame color match the existing windows.

Watch Out for Old Paint: Older homes might have lead-based paint. If you’re scraping or sanding, take precautions.

Don’t Rush It: This isn’t a hard project, but rushing can lead to gaps or damage. Take your time, especially when sealing and fitting the new sash.

Final Thoughts

Replacing a TDL sash with an SDL sash is a great home improvement move, especially if you’re looking to cut energy costs while keeping the character of your home intact. It’s a job that many homeowners can handle with some basic tools and patience. Just make sure to measure carefully, take your time, and double-check the fit before sealing everything in.